DEPTH-DEPENDENT CONFINED COMPRESSION MODULUS OF FULL-THICKNESS BOVINEARTICULAR-CARTILAGE

The objective of this study was to determine the equilibrium confined compression modulus of bovine articular cartilage as it varies with de pth from the articular surface. Osteochondral samples were compressed by 8, 16, 24, and 32% of the cartilage thickness and allowed tl, equil ibrate. Intratissue displacement within the cartilage was measured wit h use of fluorescently labeled chondrocyte nuclei as intrinsic, fiduci al markers. Axial strain was then calculated in nine sequential 125 mu m thick cartilage layers comprising the superficial 1.125 mu m and in a 250 mu m thick layer of cartilage adjacent to the cartilage-bone in terface. Adjacent osteochondral cores were also tested in confined com pression to determine the equilibrium stresses required to achieve the same levels of compression. Stress-strain data for each layer of each sample were fit to a finite deformation stress-strain relation to det ermine the equilibrium confined compression modulus in each tissue lay er. The compressive modulus increased significantly with depth from th e articular-surface and ranged from 0.079 +/- 0.039 MPa in the superfi cial layer to 1.14 +/- 0.44 MPa in the :ninth layer. The deepest layer , 250 mu m thick, had a modulus of 2.10 +/- 2.69 MPa. These moduli wer e markedly different from the apparent ''homogeneous'' modulus for ful l-thickness cartilage (0.38 +/- 0.12. MPa) and ranged from 21 to 560% of that value. The relatively low moduli and the compression induced s tiffening of the superficial layers suggest that these layers greatly affect the biomechanical behavior of cartilage, such as during confine d compression testing. The delineation of the depth-dependent modulus provides a basis for detailed study of the relationship between the co mposition, structure, and function of cartilage in such processes as a ging, repair, and degeneration.